Angiotensin II Binding Sites on Isolated Rat Renal Brush Border Membranes*

There is evidence that angiotensin II has a direct effect on reabsorptive processes of the kidney that are mediated by angiotensin II receptors on the proximal tubules. These receptors have not previously been localized to luminal brush border or basolateral membranes. The present study is the first to characterize the angiotensin II receptor of rat renal brush border membrane vesicles. Results are reported as means +/- SE Specific binding of 125Iiodo-angiotensin II at steady state was significantly increased by 100 mM NaCl or 10-25 mM MgCl2. NaC1 caused a significant decrease in the Kd from 9.5 +/- 1.3 nM (n = 2) to 6.0 to 0.3 nm (n = 3). In contrast, MgCl2 had no effect on Kd but significantly increased the binding site concentration from 162 +/- 76 (n = 2) to 597 +/- 26 fmol/mg protein (n = 3). The effects of these salts were additive. Steady state binding was achieved within 10-15 min at 24 C. Scatchard analyses of binding inhibition data indicated a single class of high affinity sites with Kd similar to that for angiotensin II receptors in known target tissues. Binding was reversible, proportional to membrane protein concentration, and did not consist of uptake into the vesicles. Octapeptide and 2-8 heptapeptide analogs of angiotensin competed for these binding sites with potencies that correlated with their binding inhibition potencies at known extrarenal and renal target tissues. The radioactivity, eluted from membranes with acid after steady state was achieved, was 57 +/- 8% (n= 4) of intact 125Iiodo-angiotensin II, as determined by rebinding to fresh membranes. This observation is inconsistent with binding to a degradative enzyme. These findings indicate the presence of high affinity specific binding sites for 125Iiodo-angiotensin II in renal brush border membranes.